1. Field of the Invention
The present invention relates to an optical device and a projector having the optical device.
2. Description of Related Art
Conventionally, a three-plate projector including a color-separating optical system for separating a light beam irradiated by a light source lamp into three color lights of R, G and B with a dichroic mirror, a three optical modulators (liquid crystal panel) for modulating the separated light beam per each color light in accordance with image information, and an optical device having a cross dichroic prism for combining the light beam modulated by the respective liquid crystal panels has been used.
In an optical device of such a projector, in order to accurately combine the light beam irradiated by the three liquid crystal panels, the three liquid crystal panels are mounted on the cross dichroic prism with high accuracy so that the corresponding picture elements of the three liquid crystal panels are accurately aligned.
Polarization plates for transmitting only the incident light beam in a direction along the polarization axis and for absorbing the light beam in the other direction and irradiating as a predetermined polarized beam, are respectively provided on the incident-side and irradiation-side of the liquid crystal panel. The polarization axis of the incident-side polarization plate and the irradiation-side polarization plate are orthogonal with each other, so that the modulated image light in accordance with image information is formed by the liquid crystal panel.
Such polarization plate ordinarily has a resin polarization film adhered on a glass base plate and such polarization film is likely to be deteriorated (e.g. distorted) on account of rise in temperature caused by absorbing the light. Especially, the irradiation-side polarization plate has to absorb all the unnecessary tight irradiated by the liquid crystal panel in projecting a full-black image onto the screen, thus being easily to be deteriorated.
Accordingly, conventional projector has a cooling mechanism for preventing the polarization film from being excessively heated by circulating cooling air thereinside by a cooling fan etc. However, since the size of the optical device is reduced in accordance with increase in illuminance and size reduction of recent projector and the optical components such as a liquid crystal panel and polarization plate are closely installed, sufficient cooling air cannot be flow through the gap between the respective optical components, so that the polarization film, especially the radiation-side polarization film cannot be sufficiently cooled. Though it is possible to increase the flow rate of the cooling air for efficiently cooling the polarization film, the size of the cooling fan or the number of revolution has to be increased, which runs counter to the requirement of size and weight reduction of projector and inevitably increases the noise.
In order to efficiently cool the polarization film of the irradiation-side polarization plate while rest noise, following arrangement has been conventionally used in an optical device (See Japanese Patent Laid-Open Publication No.2003-121931). An irradiation-side polarization plate with a polarization film adhered thereon is held on a metal holder in a thermally-conductive condition to construct a cooling mechanism. A metal base for fixing a cross dichroic prism is attached on a surface intersecting the light-incident side, and die cooling mechanism is attached on the base, where the liquid crystal panel is attached on the cooling mechanism by a pin. According to the above construction, since the heat generated by the polarization film of the irradiation-side polarization plate can be released to the base through the holder, the polarization film of the irradiation-side polarization plate can be sufficiently cooled without change flow rate of the cooling air etc.
However, in such optical device, since the liquid crystal panel is attached to the base though the cooling mechanism, when the metal base is expanded by the external heat, the metal cooling mechanism is also expanded in accordance with the heat expansion of the base, so that the spatial position of the liquid crystal panel fixed on the cooling device is changed. On the other hand, since a cross dichroic prism ordinarily is difficult to be thermally expanded as compared to a metal base, the cross dichroic prism does not follow the heat expansion of the base. Accordingly, the relative positions between the liquid crystal panel and the cross dichroic prism and, as a result, between the three liquid crystal panels may be shifted, thus causing shift between the picture elements to be combined to deteriorate the image quality of the combined image.
The same disadvantage may be not applied only to a polarization film but also found when an optical conversion film such as viewing angle compensating film and phase film having other optical function and easily causing thermal deterioration is used.